1. Field of the Invention
The present invention relates to a ring oscillator in which an oscillating signal having any frequency can be provided. Generally, in the ring oscillator that is constituted by connecting a plurality of amplifiers in multi-stages and by executing a ring feedback from the final stage amplifier to the first stage amplifier, the signal with a desired frequency can be generated by adequately combining outputs from the respective stage amplifiers with each other.
Typically, such a ring oscillator can be used for controlling the servomechanism of a video camera, the sequential operations of a computer system, various peripheral units and the like.
2. Description of the Related Art
In a known ring oscillator, usually, plural amplifiers, e.g., more than three operational amplifiers each having a relatively high input impedance are connected in a multi-stage connection (cascade connection). Further, the output terminals of a final stage amplifier are connected to the input terminals of a first stage amplifier to form a ring feedback.
In such a construction, the corresponding first output signals are issued from the respective pairs of output terminals of multi-stage amplifiers and supplied to the corresponding pairs of input terminals of the respective next stage amplifiers. Further, the corresponding branch output signals that branch from the above respective pairs of output terminals are supplied to a control switching circuit, e.g., a selector circuit, as the second output signals. Further, in the above control switching circuit, predetermined output signals are selected by switching operations from the above second output signals and then supplied to a logical circuit composed of various logical elements. If the above selected second output signals are combined adequately by the above logical circuit, the oscillating signal having a desired frequency (period) can be obtained.
In this case, as described above, the first output signals and the second output signals are sent simultaneously from each pair of output terminals of multi-stage amplifiers to the corresponding pair of input terminals of each next stage amplifier and the common control switching circuit, respectively. In other words, the above first and second output signals are taken out in parallel formation for each pair of output terminals.
Typically, a differential amplifying unit having a pair of differential transistors is arranged in the output terminals of each stage amplifier. Further, between the corresponding collectors of the above pair of differential transistors and a common power source line such as a collector voltage supply line, a pair of load elements of resistors are connected respectively. Further, a pair of connecting points between the pair of resistors and the corresponding collectors of the pair of differential transistors are connected to the corresponding output terminals respectively. Then, the first and second output signals are taken out in parallel formation from the above pair of resistors.
In the case that combinations of various kinds of second output signals are appropriately executed so that the final oscillating signal is set to a desired frequency, i.e., a desired period, if one of the second output signals corresponding to only one terminal of a pair of output terminals in a given stage amplifier is selected, then the difference between the currents that flow into the selector circuit respectively has occurred in the above pair of output terminals. Accordingly, the difference between the currents that flow through the pair of load elements of resistors respectively has also occurred and then the respectively corresponding voltage drops by the above resistors become different from each other. Therefore, the voltages of output signals between the corresponding first output signals also become unbalanced and the conditions for generating certain pulse signals from the first output signals are likely to have some discrepancies between the pair of output terminals of each stage amplifier. In other words, when combinations of a number of second output signals are modified by adequately operating the selector circuit to obtain any oscillating frequency owing to the frequency dividing operation of the logical circuit, the first output signals that flow through each stage amplifier in turn may be sometimes influenced by the voltage imbalance of the above second output signals and discrepancies between various conditions for the respective first output signals may have occurred.
Therefore, the phase lag between one side and another side of each pair of output terminals for each stage of multi-stage amplifiers becomes somewhat different, and further the time lag in the timing of the trailing edge of each pulse signal of output signals, which is originally designed to be a specified value for each stage amplifier, is shifted slightly between the above two sides of the pair of output terminals. Consequently, a problem arises in that jitters (fluctuations), etc., occur in the above trailing edge because of the time lag shifting, which leads to a deterioration of the accuracy in setting the oscillating frequency.
Particularly, in recent years, frequency control with a higher degree of accuracy has been required for video cameras, computer systems, etc. Therefore, the above problem becomes more significant.